Method of sealing chromic acid anodized aluminum surfaces



United States Patent Office 3,494,839 Patented Feb. 10, 1970 3,494,839 METHOD OF SEALING CHROMIC ACID ANODIZED ALUMINUM SURFACES Bryce Chambers, Seattle, and Sidney H. Vander Werde, Bellevue, Wash., assignors to Amchem Products, Inc., Ambler, Pa., a corporation of Delaware No Drawing. Filed Jan. 23, 1967, Ser. No. 610,808 Int. Cl. C23b 5/50, 9/02 U.S. Cl. 204-35 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the art of sealing chromic acid anodized coatings on aluminum to improve their corrosion resistance, paint adhesion, and other desirable properties. It is particularly concerned with methods of sealing for such anodized coatings which are easier to perform than those used heretofore, because the operating conditions are less critical, and which largely eliminate the sharp dependence of sealing quality on the impurity level in the sealing bath.

Anodizing is the purposeful formation, by an electrochemical process, of a protective aluminum oxide coating on the surface of the aluminum. Two types of electrolyte baths have found wide use for anodizing: Chromic acid baths and sulfuric acid baths. Because the type of electrolyte employed has an effect upon the operating conditions of the anodizing bath, and upon the properties and utilities of the anodic film, somewhat separate fields of use for these two types of anodizing have developed, although there is some over-lap. In general, chromic acid anodized coatings are thin, have little deterimental effect on the fatigue strength of the underlying metal, and are the only safe anodized coatings for use on aluminum articles having difficult-to-rinse crevices, over-lapped areas, etc. For these and other reasons, chromic acid anodizing is the preferred form of anodizing for use in the aircraft industry.

The anodic film resulting from chromic acid anodizing initially consists, in the main, of amorphous anhydrous aluminum oxide which is unstable and tends to convert to the crystalline monohydrate. It has :been found that the corrosion resistance of anodic film is enhanced, if this conversion of the anhydrous oxide to the crystalline monohydrate is intentionally performed by a method called sealing. In its simplest form, sealing consists merely of contacting the fresh anodized coating with a hot water rinse.

Simple hot water rinsing is not especially satisfactory as a sealing technique, because it generally fails to develop the maximum corrosion resistance properties which the anodized coatings have the potential for developing. For these reasons, more sophisticated hot Water sealing techniques have been developed. It has been learned that for water sealing, the purity of the water is a very important factor, and the better practice in hot water sealing has called for the use of very pure water, such as deionized or distilled water. The use of high purity water creates serious processing problems. A principal problem is the supply of the pure water itself, since few conventional water supply sources are pure enough. Furthermore even with relatively pure water, very small concentrations of some impurities, such as a few parts per million of dissolved silicon, or iron, can inhibit or prevent the sealing reaction, while other impurities become in corporated in the coating during the course of sealing and can even cause the sealed coating to have poorer corrosion resistance than an unsealed coating.

Another difficulty with purified water sealing is that the length of treatment and treating temperature must be carefully controlled. For example, if the sealing water is boiling, it may dissolve significant amounts of the aluminum oxide. It is also possible to over-seal, thus making the anodic coating slightly powdery, which is undesirable. While sealing time is thus somewhat critical, it is also lengthy, commonly taking 15 minutes or more.

Some prior workers believe that the inclusion of traces of chromic acid in the high purity water sealing baths has been beneficial, but other experience indicates that chromic acid can effectively prevent the sealing reaction, even in very small concentrations. Those who use chromic acid in such sealing baths generally seek to adjust the pH of the bath to be between 4.0 and 6.0.

We have found that superior sealing of chromic acid anodized coatings can be obtained if the sealing is carried out in an aqueous bath which contains hexavalent chromium in amounts from about 0.4 gram/liter to about 4 grams/liter (calculated as CrO and a soluble complex fluorine hearing compound in an amount from about 0.1 gram/liter and 1.0 gram/liter (calculated as fluorine), said solution having a pH between about 2.0 and 4.0, under conditions discussed more fully below.

This method has a number of advantages. A key advantage is that it is in large measure insensitive to water impurities, thus making it possible to employ ordinary tap water in the bath. This is in sharp contrast to the high purity water system of sealing. For example, good pure water sealing practice requires an impurity level of about 12 parts per million or lower, and as pointed out above, even minute concentrations of some impurities are intolerable. On the other hand, our process is quite successful when tap water of 40 parts per million impurities, including about 22 parts per million of hardness, is used, and even higher water impurity levels, in the neighborhood of 113 parts per million impurities can be well tolerated. Of course, some tap water sources have extremely high impurity levels, on the order of 1200 parts per million, and with such extremely impure Water, users of the present invention may find it necessary to alter the operating conditions somewhat or to partially purify the tap water for use in the process. In summary, the method of the present invention is substantially insensitive to water impurities in the kinds and amounts commonly encountered in tap water.

A related advantage also flows from the insensitivity of the present invention to water impurity level. A practical ly unavoidable source of water impurities in a sealing bath is the aluminum work which is processed in the bath. It carries into the bath impurities originating in earlier processing stages, and when high purity water sealing techniques are used, this factor alone necessitates frequent bath replacement even if the source of water is of good quality. Inasmuch as the process of the invention is quite insensitive to water purity, such carriedin impurities can generally be easily tolerated.

While we do not wish to limit the scope of this invention to the particular theoretical explanation of its mechanism, we believer that the desirable insensitivity of the sealing baths of the invention to Water impurities can be attributed to the fact that it appears that the chemical components of the sealing bath, rather than water, are sealed into the anodized coating. Thus, the sealed coating formed appears to be of a fundamentally different type than that obtained with water sealing, and more importantly, from a practical standpoint, the process thus is quite insensitive to water purity.

Several process advantages result from the use of the method of the invention. While sealed coatings of the best quality are produced under the preferred conditions discussed below, the process variables are not nearly so sensitive as those involved in pure water sealing. Thus the treating time is both shorter and less critical. The concentration limits for the hexavalent chromium and fluorine bearing compounds are relatively broad and quite easily maintained. The treating temperature is similarly less critical, and is also easily maintained. In this connection is should be noted that the preferred treating temperature is considerably below the boiling point of the sealing solution, and the danger of dissolving aluminum oxide is substantially eliminated.

The sealed chromic acid anodized coatings produced in accordance with the invention are superior to sealed coatings produced by the high purity water sealing process in a number of respects, as well as being superior to sealed coatings produced by the less sophisticated hot water rinse technique, and to unsealed coatings. In this connection, it should be noted that while high purity water sealed coatings are generally better than unsealed coatings in corrosion resistance, they are generally inferior in some other important respects, such as ability to bond to structural adhesives, and paint adhesion properties. For this reason, it has heretofore often been necessary to forego the improved corrosion resistance resulting from sealing, when one of these other important properties is desired.

The sealed chromic acid anodized coatings produced in accordance with the invention are superior to purified water sealed coatings in corrosion resistance. Coatings sealed in accordance with the invention consistently show very little or no corrosion after 800 to 1200 hours of exposure to salt spray.

Heretofore sealed coatings have been unsatisfactory for use with structural adhesives of the type commonly used in the aircraft industry, since they have given inadequate bond strength. We have found that the sealed coatings produced by the invention on common structural aluminum alloys such as types 2024, 7075, 7079, 7178, give bond strength approaching that obtained with unsealed anodized coatings. They at the same time have much better corrosion resistance, and when this factor is combined with their relatively good bond strength qualities, they have an over-all advantage for use with structural adhesives such as epoxy and nitrile phenolic adhesives.

Similarly, the paint adhesion properties of chromic acid anodized coatings sealed in accordance with the invention, as demonstrated by impact tests, water immersion tests, etc., are superior to those of unsealed anodized coatings, which are in turn superior to those of coatings sealed with high purity water.

As was pointed out above, the sealing solutions employed in accordance with the invention contain hexavalent chromium and a fluorine bearing compound. The term hexavalent chromium is used in this disclosure to denote chromic acid (CrO or its soluble salts, such as ammonium dichromate, potassium dichromate, sodium chromate, etc. The concentration of hexavalent chromium should be from 0.4 gram/liter to about 4 grams/liter, calculated as CrO When the concentration of hexavalent chromium is lower than 0.4 gram/liter, the performance of the sealing solution is more sensitive to the nature and quantity of impurities in the water, and unsatisfactory sealing may thus result. When the concentration of hexavalent chromium is more than about 4 grams/liter, the sealed surface may have objectionable discoloration, and may be somewhat powdery. Within the above mentioned concentration range for hexavalent chromium there is a preferred concentration range of from about .60 to about .85 gram/liter, calculated as CrO Within this range, which is readily maintainable. the best sealing results are obtained, and the effect of water impurities is minimized.

The term fluorine bearing compound is used in this disclosure to denote a complex fluoride such as fluosilicic acid, fiuoboric acid, fluostannic acid, fluotitanic acid, and the soluble salts of any of them. The fluorine bearing compound should be present in an amount between 0.1 gram/liter and 1.0 gram/liter, calculated as fluorine. The considerations underlying this concentration range are similar to those for the hexavalent chromium component. Below 0.1 gram/ liter of fluorine bearing material the quality of the sealed coating may be adversely affected by impurities in the water, and above the high concentration limit given, the sealed coatings may have objectionable discoloration and may show powderiness. Within the above mentioned concentration range a preferred range for the fluorine bearing compound, expressed as fluorine, is between about 0.15 gram/ liter and about 0.25 gram/liter. Within this range, the best quality sealed coatings are obtained and the effect of water impurities is minimized.

The pH of the sealing solution should be between about 2.0 and about 4.0 in order to maximize the performance properties of the sealed coating. The sealing solution can readily be formulated with sources of hexavalent chromium and fluorine bearing compounds which yield a pH within the above range. However, if desired, the pH may be adjusted with mineral acids, such as nitric or hydrochloric or with bases such as sodium hydroxide or ammonium hydroxide.

In connection with the above mentioned concentration ranges for the hexavalent chromium and the fluorine bearing compound, it should be observed that when the concentration of the fluorine bearing compound is relatively low, then the concentration of hexavalent chromium should also be relatively low, and conversely, when the concentration of the fluorine bearing compound is high, the concentration of hexavalent chromium should also be relatively high. By following this general rule within the above stated limits, assurance is attained that the coaction of the materials in the sealing solution will yield good quality sealed coatings.

The freshly anodized coatings are sealed in accordance with the invention by immersing them in a bath made up as described above for periods from about 3 to about 8 minutes. If desried, somewhat shorter sealing periods, on the order of one to two minutes, may be used, but the corrosion resistance of the sealed coatings is somewhat less. Similarly, longer sealing periods, such as 9 to 10 minutes, may be used, but there is an increased chance that objectionable staining or powderiness of the coating may occur.

The treating temperature of the sealing bath is preferably maintained between about and F. Once again a somewhat broader temperature range, from about 80 F. to about F. can be used if desired, but the quality of sealed coatings produced is likely to be somewhat inferior. The above mentioned narrower temperature range is convenient, and is easily maintainable; thus it is preferred for optimum results.

The following is an example of a sealing operation conducted in accordance with the invention. This example will include also a brief outline of the complete processing procedure for aluminum, so that the general context in which the invention finds employment will be clear.

The aluminum to be treated is first cleaned in an alkaline cleaner of the kind commonly used in the aluminum treating art, and is then deoxidized to remove the naturally occuring oxide film from the surface, by the use of deoxidizing solutions and procedures which are also familiar in the art.

The aluminum surface is then treated in a chromic acid anodizing bath, following standard good procedure for the operation of such a bath. One commonly used anodizing treating cycle consists of 30 to 45 minutes treatment at 40 volts in a bath containing 100 grams/ liter total CrO at 95 F. The treating voltage is usually increased from a low starting voltage to nominal treating voltage during the first few minutes of treatment. Another cycle which gives good uniformity of coating weight and color on structural alloys involves the use of 22 volts for 30 minutes, again including a gradual voltage build-up at the outset.

The freshly anodized surfaces are removed from the anodizing bath and given a thorough water rinse.

The anodized surfaces are then immersed in a sealing hath made up of the following materials:

Tap water (impurity level approximately 40 p.p.m.) pH

Balance 2.04.0

The immersion time for the surfaces is 3 to 8 minutes and the sealing solution is maintained at a temperature of 140 to 150 C.

Following the sealing step, the sealed aluminum surfaces are removed from the sealing bath and are thoroughly rinsed with water, at a temperature below about 100 F. We have found the use of a water rinse to be advantageous in that it enhances the performance proparties of the sealed coating. It is preferred that the rinse water be below 100 F. because use of higher temperatures tends to cause undesirable staining of the surface. Following rinsing the work is air dried.

Aluminum surfaces treated in accordance with the above procedure have consistently shown little or no corrosion in salt spray for periods of 800 to 1200 hours, and have good paint adhesion properties as well as improved bonding properties when used with structural adhesives.

We claim:

1. A method for sealing a chromic acid anodized coating on aluminum comprising immersing a freshly anodized and rinsed aluminum surface in an aqueous sealing solution consisting essentially of hexavalent chromium in an amount from about 0.4 g./l. to about 4.0 g./l calculated as CrO and a soluble complex fluorine bearing compound in an amount from about 0.1 g./l. and 1.0 g./l., calculated as fluorine, said solution having a pH between 2.0 and 4.0, said surface being immersed in said solution for about 3 to about 8 minutes, and thereafter removing said surface from said solution.

2. A method in accordance with claim 1 in which the which the concentration of said hexavalent chromium is between about 0.60 and about 0.85 g./l., calculated as (1rO and the concentration of said soluble complex fluorine bearing compound is between about 0.15 and about 0.25 g./l.

3. A method in accordance with claim 1 and further comprising rinsing the sealed surface with Water having a temperature below F.

References Cited UNITED STATES PATENTS 2,040,617 5/1936 Mason et al 204-58 XR 2,493,934 1/1950 Waring 1486.2 2,507,956 5/1950 Bruno et a1. 148-6.2 2,851,385 9/1958 Spruance et al. 148-6.2 2,868,679 1/1959 Pimbley 1486.2 2,927,872 3/1960 Cohn 1486.1 3,210,316 10/1965 Merck et al 1486.27 3,397,090 8/1968 Champaneria et al. 148-6.2 3,410,707 11/1968 Pocock et al 106-14 OTHER REFERENCES Chemical Abstracts, vol. 49, 15719b, Sogi, Anticorrosive Treatment of Aluminum and Its Alloys.

JOHN H. MACK, Primary Examiner W. B. VANSISE, Assistant Examiner US. Cl. X.R.

Patent No.

Dated February 10 1970 Inventor(s) Bryce Chambers at al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 31, "hearing" should read -bear-ing-.

Column 5, line 29 "150C" should read --l50F.--.

Column 6 Claim 2 line 2 delete -which the-.

(S Attest:

Edward Mllmhmln Attes ting Officer SIGNED AND SEALED JUN 231970 WIMIMET QMIIIER, JR. Gomissioner of Patents 

